A hacker, using Twitter handle CyberZeist, has claimed to have hacked the FBI's website (fbi.gov) and leaked personal account information of several FBI agents publically.

CyberZeist had initially exposed the flaw on 22 December, giving the FBI time to patch the vulnerability in its website's code before making the data public.

The hacker exploited a zero-day vulnerability in the Plone CMS, an Open Source Content Management software used by FBI to host its website, and leaked personal data of 155 FBI officials to Pastebin, including their names, passwords, and email accounts.

CyberZeist tweeted multiple screenshots as proof of his claims, showing his unauthorized access to server and database files using a zero-day local file inclusion type vulnerability affecting its python plugins.

Hacker also found that the FBI's website is hosted on a virtual machine running a customized older version of the open-source FreeBSD operating system.

According to another tweet, the Plone CMS zero-day exploit is up for sale on an unnamed dark web marketplace.Not Just FBI, All Sites Using Plone CMS are Vulnerable

The Plone CMS is considered to be one of the most secure CMSes available today and is used by many major websites like Google, and major United States agencies including the FBI and the CIA.

CyberZeist also warned other agencies, including the European Union Agency for Network and Information Security, Intellectual Property Rights Coordination Center, and Amnesty International, which are currently using the Plone CMS that they too are vulnerable to a similar attack.

The hacker also claimed the FBI officials contacted him and requested a copy of the stolen credentials, which they declined to provide.

The FBI authorities have yet to respond to the claims.

This is not the first time CyberZeist claimed to have hacked the FBI website. In 2011, the hacker breached the FBI website as a member of the infamous hacker collective known as "Anonymous."